1. Field of the Invention
The present invention relates to an optical waveguide device having optical waveguides and using a piezoelectric substrate that imparts an electro-optical effect to light signals. More particularly, the invention relates to an optical waveguide device suited to a Mach-Zender-type optical modulator.
2. Description of the Related Arts
An optical waveguide device, a kind of the Mach-Zender-type optical modulator, capable of separately controlling the phase of light signals propagating along a first and a second waveguides extending in parallel relationship is known as a Dual-Drive-type optical modulator conducting an optical intensity modulation by controlling the phase of the light signals propagating along the first and second waveguides with external high-frequency electrical signals and is used in a high speed long-distance optical communication transmission system.
In such a Dual-Drive-type optical modulator, separate control is provided of the phase of light signals propagating along a first and a second optical waveguides (for convenience, each may be referred to as “arm”) extending in parallel relationship formed on a piezoelectric substrate having an electro-optical effect, by use of external electrical signals. That is, it is possible to separately control the refractive index of the optical waveguide of each arm.
FIG. 1 is a schematic view illustrating the principle of such a Dual-Drive-type optical modulator.
In FIG. 1, a first and a second optical waveguides 10, 11 are formed on a piezoelectric substrate 1 and a light signal from an optical input section (OPT-in) is branched and input commonly to the optical waveguides. The light signals propagated on the first and the second waveguides are coupled at and output from an optical output section (OPT-out).
Progressive wave electrodes 20, 21 are provided in the proximity of each arm of the optical waveguides 10, 11 such that electric fields are efficiently applied to the first and the second waveguides 10, 11. That is, one of the progressive wave electrodes 20 (21) controls the phase of the light signal propagating along one arm 10 (11) and the other progressive wave electrode 21 (20) controls the phase of the light signal propagating along the other arm 11 (10).
In order to realize a meaningful optical modulation using an optical modulator as above, it is necessary to input, synchronizing the timing, a modulation signal (for example, DATA=Q and the inversion of the DATA=/Q) in the microwave band correlated to each of the progressive wave electrodes 20, 21.
FIG. 2 illustrates an example of V1=Q and V2=/Q as the modulation signals in the microwave band input to the progressive wave electrodes 20, 21 in FIG. 1. FIG. 3 illustrates an optically modulated light signal output in response to the phase difference between these modulation signals V1=Q and V2=/Q.
Then, usually, in the Dual-Drive-type optical modulator, the progressive wave electrode 20, 21 are formed in parallel with respectively the optical waveguides 10, 11 as shown in FIG. 1. In addition, input sections 20-1, 21-1 are arranged on the same side as the optical wave 10, 11 as strip lines.
Furthermore, the space between the input sections 20-1, 21-1 constituted by the strip lines of the progressive wave electrode 20, 21 is arranged being different from the space between the electric connectors for inputting a microwave as a modulation signal. The electric field applied to the light signal propagating each of the arms interacts with each of the light signals with a different phase due to the relative phase relation of the synchronized input microwave at the electric connectors as shown in FIG. 2 produced by the difference between these spaces.
In order to avoid the above, delay lines for correcting the timing (i.e., routing structures of the strip lines) are provided on the piezoelectric substrate 1 taking into consideration the difference between the space between the input sections 20-1, 21-1 and the space between the electric connectors.
In the above conventional structure, the length of the delay lines provided on a chip are determined in proportion to the mounting pitch of the electric connectors on a housing. In this case, if the dielectric constant of the piezoelectric substrate 1 is large, then, the attenuation for the microwave over the length of the delay lines provided on the piezoelectric substrate 1 for adjusting the timing to reach the interaction region with the light signal becomes large. This is a factor which cause a bandwidth degradation of the optical modulator.